By amalgamating various canal companies, the Grand Union Canal was formed to link London with the Nottinghamshire/Derbyshire coalfields and Birmingham under single ownership. The main line of this canal was the Grand Junction Canal (Braunston to Brentford), which was authorised in 1793 to provide a more direct route to London, than the then existing route of the Oxford Canal and the Thames.
Construction of the Paddington Arm was authorised in 1795 and it was completed in 1801.
The Slough Arm was the last canal to be built in Britain except for the Manchester Ship Canal. The canal was started to be built in early 1880. It was completed an opened on 4th December 1882. The canal was used to transport the bricks from the various Slough brickworks. The canal was also used for the establishment of various sand, brick and gravel wharfs along the line. The existing brickworks expanded their production and transported bricks to many parts of London. Traffic on the canal grew rapidly and reached a peak in 1905 when over 192,000 tons were carried in that year. The Brick and Gravel trade continued throughout the 1920’s and 1930’s and also through the War Years but, by then, the clay and gravel deposits were becoming worked out. Traffic of waste from London to infill the redundant pits continued for a while as did the delivery of timber to Slough Wharf. The last recorded commercial cargo was delivered in March 1960.
There is a detailed history of the Grand Union Canal at Wikipedia.